The emphasis of much of robotic development has been on structures intended for durability, to apply force, to operate in human-unfriendly or constrained environments, and to move at high speed. Many of the structures of hard robots are based on structures derived from the body plans of mammals (or parts of them). Their skeletons are typically rigid, and electric motors (or sometimes hydraulic or pneumatic systems) provide activation. So-called “hard” robots—robots based on rigid structural elements, typically of metal and conventional mechanical joint bearings, and actuators—are highly evolved for operations in controlled environments (e.g., in manufacturing). They are, however, often heavy and not well adapted for unstructured, unstable, or fluid environments (e.g., loose gravel, sand, or mud). Robots used for performing delicate tasks (e.g., surgery) can be flexible in their movement, but are quite specialized. Airborne robots (e g, unmanned and autonomous air vehicles) are highly evolved, but do not have to deal with the vagaries of rough terrain.
“Soft” robots—robots fabricated using flexible or elastomeric structural elements—offer potentially useful approaches to overcome the challenges faced in hard robotics. They can be designed to have a low center of gravity. They can also distribute pressure evenly on the ground, or with the objects with which they interact. They can use their often highly non-linear responses of actuation to accomplish, relatively simply, types of motions and tasks (e.g., grasping soft objects) that would be very difficult to accomplish with hard robots and conventional controllers.
Pneumatic soft robots—such as those driven by micro-pneumatic actuators embedded in elastomeric materials—offer an underexploited entry into the family of soft robots and soft machines. For pneumatic actuation to be most useful, it should satisfy three conditions. i) It should be flexibly controllable in direction and force. ii) It should take advantage of its non-linearities to simplify the accomplishment of functions that are difficult with linear actuators. iii) It should be easily incorporated into designs that are practical to fabricate, inexpensive, and functional.
Muscle—a structure ubiquitous in nature—still has no real counterpart in materials science, robotics, or actuation; although there are examples of electromagnetic actuators and other useful structures (for example “air muscles”) which have some muscle-like properties. There are a number of approaches exploring muscle-like structures as new types of components for soft robotics are developed, but few have been widely developed or deployed.
Many biologically inspired soft robots have been built using electroactive polymers (EAPs) actuators. For example, Nie et al. (SPIE 2007, 6423, 64232A) developed a tortoise-like flexible micro robot that can crawl and swim underwater by using legs actuated by an ionic conducting polymer film (ICPF). The response of systems based on EAPs to control signals are typically slow (unless they are very small) since they require the diffusion of ions for their flexible structures to work.
Unless intrinsically anisotropic in structure (e.g., liquid crystals) or processed (e.g., by stress orientation) to generate anisotropy, organic polymers are usually approximately isotropic in their response to stress, and thus constrained in the range of motion they can generate on actuation. New materials and structures that are easy to control, inexpensive, and compatible with soft actuation and soft robotic applications would expand the capabilities of this area of functional materials. Elastomeric polymers will be an important class of materials for soft robotics.